You might think to be better at cross-country running you should stick to running, but avoiding weight training puts you at a disadvantage. Lifting weights enhances your upper-body strength, so your arms don't fatigue and fall into a poor position during a run. Train your legs to be strong so you run more efficiently, have more power and, ultimately, demonstrate greater speed. Always consult your doctor (and coach) before beginning this or any exercise regimen.
Terrain Technique
During cross country, you encounter a variety of terrains, from dirt to asphalt to trail to gravel, and must navigate through quick turns, steep inclines, bridges, creeks and sometimes obstacles along the trail. Building strong thighs, glutes and hips helps you power over these challenges as improves your stamina so you don't fatigue after encountering them.
When to Train
Preseason is the optimal time to include strength training. Your running mileage is lower, so you can hit the weights harder and build up stamina and strength for the season to come. A study published in a 2013 issue of "Medicine and Science in Sports and Exercise" did find that heavy weight training was particularly effective for female runners and that male runners may have their run times negatively affected by heavy resistance training performed during competition season. A low-volume, less frequent weight-lifting regimen may be included during in-season to maintain strength. Again, check with your coach to determine the best regimen for you.